
\begin{tikzpicture}
  \coordinate[label=above:$A$] (A) at (4,3);
  \coordinate[label=left:$B$] (B) at (0,0);
  \coordinate[label=right:$C$] (C) at (4,0);
  \coordinate[label=below:$A'$] (A') at (4,-3);
  \pic[mark angle={green}{3mm}{1.2}] {right angle=B--C--A'};
  \draw (B) -- node[above left] {$c$} (A)
    -- node[right] {$b$} (C)
    -- node[below] {$a$} cycle;
  \draw[dashed] (C) -- node[right] {$b$} (A')
    -- node[below left] {$c$} (B);
\end{tikzpicture}
